Various ways are recognized in the photographic industry for releasing a PUG from a compound, such as a coupler, in a photographic material and process. Release can be direct, for example upon reaction of the coupler with oxidized color developing agent during processing, or it can be indirect through a linking or timing group. Linking and timing groups provide the ability to control the timing and rate of release of a PUG in a photographic element, as well as the rate and distance of diffusion of the PUG in the element during processing.
U.S. Pat. No. 4,248,962 describes compounds that release a PUG, such as a development inhibitor group, through a timing group which functions by (intramolecular) nucleophilic displacement reaction. Other examples of compounds that are capable of releasing a PUG are described in European Patent Application 0 514 896 and in U.S. Pat. Nos. 4,409,323 and 4,861,701. In European Patent Application 0 514 896, compounds are described which release a PUG through a timing or linking group which functions by electron transfer down an unconjugated chain. In U.S. Pat. No. 4,409,323, compounds are described which release a PUG by a mechanism which involves electron transfer down a conjugated chain. In U.S. Pat. No. 4,861,701, sequences of timing groups are utilized to release a PUG and to provide desirable control over the impact of the PUG on photographic properties.
European Patent Applications 0 499 279 and 0 438 129 describe photographic couplers which allegedly provide superior release rates over compounds described in the aforementioned U.S. Pat. No. 4,861,701. Specifically, these applications describe photographic couplers having a heterocyclic timing nucleus (typically pyrazole) attached to a coupler moiety through an --O--C(O)-- or --OCH.sub.2 -- group, or other group capable of releasing the heterocyclic timing nucleus by electron transfer down an unconjugated chain. A development inhibitor moiety is attached to the 4-position of the heterocyclic timing nucleus and is released therefrom upon reaction of the couplers with oxidized color developing agent during processing.
The compounds described in the above European Patent Applications do not provide for any flexibility in their rate of release of a PUG, or in their synthetic design, as they are limited by the presence of the particular first timing or linking group. More importantly, though, it was determined that the compounds in both of these applications exhibit poor stability and decompose when stored for prolonged periods under tropical conditions. Thus, they are of limited practical value in today's photographic industry.
The present inventors therefore sought to improve upon the couplers described in the aforementioned European Patent Applications and to provide alternative means by which to release photographically useful groups in a controlled manner to photographic emulsions. In so doing, the inventors synthesized couplers having pyrazole timing groups wherein the PUG was attached through other than the 4-position. It was determined, however, that although such couplers were generally stable when stored upon tropical conditions for prolonged periods of time, and were synthetically simple to manufacture, they exhibited a relatively low photographic activity--that is, they failed to release their PUG at an efficient rate. An effort was therefore undertaken to determine how the activity of these couplers could be improved.